Abstract

The human intervertebral disc is a highly inhomogeneous fiber composite pressure vessel. If damaged in the post-infant stage, the disc does not repair and may need surgical correction. Diagnosis of whether a disc is damaged is difficult at best and, at this time, involves invasive techniques which are not free of danger (injection of x-ray opaque liquids). Starting from a desire to develop a noninvasive diagnostic technique based on x-ray and computer aided image enhancement, we became interested in the mechanical properties of the disc. These would be important in gaging the x-ray detected deformations of the disc under various loads. During the course of this work we became aware of another need of diagnostics related to estimating the proclivity of an intact disc to sustain damage under unusual loads. It turns out that the water content of the disc material dominates its mechanical behavior. Since modern medical equipment such as the EMI-x-ray body-scanner may record quantitatively the water content of the internal body organs, the possibility exists to gage in-vivo water content measurements with the mechanical performance of discs.
Because the layers of the disc's annulus fibrosous are so thin, we have had difficulty in preparing single-layer specimens. So far, we have worked mainly with three-layer specimens. That test geometry has been sufficient to establish several important aspects of the mechanical properties.
We find that the relaxation behavior is very sensitive to moisture content. Accordingly we have worked with carefully controlled environments including saline solutions claimed to represent body conditions. A major difficulty in obtaining repeatable results is obtaining straight test specimens and holding them in the clamps of the testing apparatus. If moisture and temperature conditioning was repeated without reclamping the specimen, reasonably repeatable results were obtained. No aging was observed after repeated drying and moisturizing cycles.
We also found that water diffuses slowly in the layers. The water apparently acts similar to the solvent in a polymer, effecting a change in the relaxation times. Increasing water content causes shortening of relaxation times, drying having the opposite effect. Upon controlling the water content of the specimen we are thus able to measure the relaxation behavior in various time domains. Data covering a wide spectrum of relaxation times is presented which includes all of the time scales experienced by the human body. This mechanical characterization gives us an estimate of how discs respond to different rates of deformation and loading conditions.
It is of interest to note that with age (past age 30) the moisture content of the human disc decreases (possible other changes involving increased cross-link density of the mucopolysaccharides as well as an exchange of mucopolysaccharides for collagen). As a result one would expect the human intervertebral disc to react more stiffly with increasing age under nearly constant speeds of motion. Combining this observation with the changes in the vigor of motion/muscle activity as a function of age allows a tentative explanation of the statistic that the largest incidence of disc problems occur around age 40-50.